THE PRESSURE GRADIENT FORCE IN SIGMA-CO-ORDINATE OCEAN MODELS

摘要

The error in computing the horizontal pressure gradient force near steep topography is investigated in a primitive equation, -co-ordinate, numerical ocean model (Blumberg and Mellor, in Three -Dimensional Coastal Ocean Models, Vol. 4, American Geophysical Union, Washington D.C., 1987, pp. 1-16). By performing simple test experiments where the density field is allowed to vary in both the vertical and the horizontal direction, severe errors are detected in the areas where the isopycnals hit the sloping bottom. An alternative method of computing the pressure force (Stelling and van Kester, Int. j. numer. methods fluids, 18, 915-935 (1994) is adopted, resulting in substantial reduction of the errors. However, a systematic underestimation of the calculated quantities is revealed, leading to erroneous depth-mean values of the pressure force. In this study a modification of the Stelling and van Kester method is proposed which seems to improve the overall performance of the method.